Anaerobic digestion is a biological process which can be used to convert inexpensive, readily available and inexhaustable resources, notably sewage sludge, industrial and municipal waste and biomass material to valuable product gases, principally methane. Anaerobic digestion has been used commercially for the treatment and stabilization of organic wastes for a century. Almost every municipality disposes of most of its sewage sludge by anaerobic digestion. In addition, many industries treat solid and liquid waste streams in anaerobic treatment facilities. The process has advantages, not only in its suitability for treating wastes having a high water content or difficulties in incineration, but also facilitates the recovery of methane as a source of clean energy. A great deal of effort has gone into designing anaerobic digestion systems which operate reliably, efficiently and quickly. Among the designs developed for anaerobic digestion systems are the completely mixed reactor, the anaerobic contact process, the upflow and downflow packed bed reactors, the fluidized bed reactor, the upflow anaerobic sludge blanket, the baffled reactor, and various two stage processes. (The apparatus employed in carrying out the anaerobic digestion processes are referred to as reactors or digesters.)
All of these processes are dependent upon the same anaerobic microorganisms to perform the conversion of the organic matter to methane. While Applicant does not wish to be bound by any particular theory or hypothesis with regard to this discussion or any other throughout this application, anaerobic digestion of organic matter to methane and carbon dioxide is presently thought to occur in three stages, each of which involves a different type of bacteria.
The first stage of anaerobic digestion involves the fermentative bacteria. Fermentative bacteria hydrolyze the polymers of the primary substrates, such as polysaccharides, proteins and lipids, and ferment them to produce fatty acids (such as propionate and butyrate), organic acids, alcohols, ammonia, sulfide, CO.sub.2 and H.sub.2.
The second stage of anaerobic digestion is the degradation of the fatty acids, some organic acids and alcohols produced in the first stage. These compounds are degraded by a second group of bacteria called the obligate H.sub.2 -producing (i.e., proton-reducing) acetogenic bacteria. Only a few of these microorganisms have been isolated and studied. The products of the second stage degradation include acetic acid, H.sub.2 and CO.sub.2.
The third and final stage of anaerobic digestion involves the methanogenic bacteria. Some methanogenic bacteria cleave acetic acid to product CO.sub.2 and CH.sub.4. This reaction is very important because approximately 70% of the methane produced during anaerobic digestion is derived from the methyl group of acetic acid. Other methangenic bacteria utilize the H.sub.2 produced by other microorganisms to reduce CO.sub.2 and CH.sub.4.
Depending on the content of the waste material fed into an anaerobic digester, different stages may be the rate-limiting step in the digestion process. In the digestion of soluble compounds, the rate-limiting step has been identified as the third stage or methanogenesis, and more specifically, the aceticlastic (uses acetic acid) methanogenic step. For the digestion of insoluble wastes, the hydrolysis of these materials, or the first stage of anaerobic digestion, may be the rate-limiting step.
Hydrogen, which is involved in many principal biological reactions, is recognized as being the controlling influence on the overall scheme of waste utilization. Hydrogen exerts control at certain points of anaerobic digestion.
Hydrogen plays an important role in regulating the proportions of the various products produced by the fermentative bacteria. During fermentation, H.sub.2 is produced from electrons generated in the oxidation of reduced pyridine nucleotides: EQU NADH+H.sup.+ .revreaction.H.sub.2 +NAD.sup.+ .DELTA.G.degree.'=18.0kj/reaction
H.sub.2 formation is favored only when the partial pressure of H.sub.2 is very low, as it is when H.sub.2 is effectively metabolized by methanogens during the third stage of anaerobic digestion. At low partial pressures of H.sub.2, the flow of electrons (NADH) generated during glycolysis is toward the reduction of protons, resulting in H.sub.2 formation. The H.sub.2 allows pyruvate to be degraded to acetate, CO.sub.2 and H.sub.2. When the partial pressure of H.sub.2 is increased, the flow of electrons from NADH shifts from H.sub.2 production to the formation of reduced electron sink fermentation products, such as propionate and longer-chained fatty acids, lactate or ethanol from pyruvate. Therefore, in ecosystems in which methanogens are effectively utilizing H.sub.2, fermentative bacteria produce more acetate, CO.sub.2 and H.sub.2, but produce little or no ethanol or lactate and produce considerably less propionate and butyrate.
Hydrogen also exerts control during the process in which acetogenic bacteria act on fatty acids, organic acids and alcohols to produce acetic acid and hydrogen. The equilibrium reactions for the degradation of butyrate and propionate under standard conditions are: EQU CH.sub.3 CH.sub.2 CH.sub.2 COO.sup.- +2H.sub.2 O.revreaction.2CH.sub.3 COO.sup.- +H.sup.+ +2H.sub.2 EQU .DELTA.G.degree.'++48.1 kJ/reaction EQU CH.sub.3 H.sub.2 COO.sup.- +3H.sub.2 O.revreaction.CH.sub.3 COO.sup.- +H.sup.+ +2H.sub.2 EQU .DELTA.G.degree.'=+76.1 kJ/reaction
As indicated by large positive values for .DELTA.G.degree.', these reactions do not favor degradation. However, when the partial pressure of H.sub.2 is maintained at a very low level in the digester, H.sub.2 production from these compounds is thermodynamically favorable. For example, when the partial pressure of H.sub.2 is decreased below 0.15 atm, the degradation of ethanol becomes energetically favorable. However, the degradation of butyrate or propionate is not energetically favorable until the partial pressure is lowered to about 2.times.10.sup.-3 atm or 9.times.10.sup.-3 atm, respectively. Thus, a slight increase in the partial pressure of H.sub.2 will halt the degradation of these compounds, with propionate degradation being the first reaction to be inhibited. As expected, studies have shown that during digester failure, propionate is the first acid that accumulates.
Thus, as discussed above, the partial pressure of H.sub.2 not only regulates the proportion of end products of the fermentative bacteria in stage one but also the degradation of these fermentation products by the H.sub.2 -producing acetogenic bacteria in stage two.
Methanogenic bacteria, in addition to producing gaseous fuel (methane) in stage three, can be used to maintain the H.sub.2 concentration in the reactor at a very low level and thus allow the digestion of organic matter to proceed efficiently. Therefore, to obtain maximum performance from an anaerobic digester, the concentration of methanogens within the digester should be maximized. Several methods have been used to obtain high concentrations of bacterial biomass (including high concentrations of methanogens) in anaerobic digesters. These methods include: (1) the recycle of solids in the anaerobic contact process; (2) attachment of biomass to supports in fixed films and fluidized or expanded beds; and (3) formation of granular biomass in a sludge blanket. While these methods have been successful in achieving high bacterial concentrations, they do not increase th population of methanogens in proportion to the other bacteria. Hence, the H.sub.2 partial pressures are not regulated in these systems and rates are not maximized.
The reaction kinetics of anaerobice digestion are quite slow. Reaction times of up to 30 days may be required to efficiently stabilize and reduce sludge volumes and/or reduce the Biological Oxygen Demand (BOD) to acceptable levels. In addition, the start-up of anaerobic digesters for sludge treatment is a slow and uncertain process. It may take several months after inoculation before a digester is operating at its desired capacity. In some cases, for example, in a very large fixed film reactor, it may take up to one year for the anaerobic biomass to develop sufficiently to attain maximum digestion efficiency.
Once a digester has achieved maximum efficiency, if the digester is shocked, for example, by shortening the retention time, overloading the system or adding a toxic substance, recovery can also be very slow. As anaerobic digestion is presently practiced, it may take several months to recover from the resulting lowered conversion efficiency or complete inhibition of conversion and regain the conversion efficiency which existed before the upset.